Conventionally, in order to achieve a high level of automated operation in a plant, a factory, or the like, a distributed control system (DCS) is implemented. In a distributed control system, on-site devices (measuring instruments and actuators), which are referred to as field devices are connected via a communication means to a control apparatus that controls the field devices. Although the communication system forming the base of such a distributed control system had almost always communicated by cable, recent years have seen the implementation of communication systems that communicate wirelessly, in conformance with an industrial wireless communication standard such as ISA100.11a or WirelessHART (registered trademark).
A wireless communication system conforming to these wireless communication standards uses TDMA (time-division multiple access) as the media access method. In this wireless communication system, a management device called a system manager (or a network manager), which manages communication resources (channels, time slots, and the like) in the TDMA system, is provided. Specifically, the management device creates a communication schedule, in which time slots and channels mutually differing between each wireless communication performed via a wireless communication network are allocated, and manages the communication resources so that allocations of communication resources do not overlap.
In this case, field devices (wireless field devices) used in the above-noted wireless communication system are basically installed singly in a plant or the like and cannot receive electrical power supplied from outside (for example, electrical power supplied via a communication bus). For this reason, batteries are used as power sources for field devices and, in order to reduce the power consumption of the field devices as much as possible, power-saving operation is done in accordance with a communication schedule created by the above-noted management device. Specifically, when a field device does not need to communicate, it goes into the sleep state. The sleep state is a state in which a field device is joined to a wireless network but does not transmit or receive a wireless signal, so as to reduce the power consumption as much as possible. Only if the need arises for a field device to communicate, does it transition to the active state, and use communication resources allocated by the management device to transmit or receive a wireless signal. The active state is the state in which a field device can transmit or receive a wireless signal.
In the above-noted wireless communication system, a prerequisite for communication to be performed normally in accordance with the communication schedule created by the management device is that the times of wireless devices (including wireless field devices) performing wireless communication via a wireless network are mutually synchronized. For this reason, in a wireless communication system, downstream wireless devices (for example, wireless field devices) periodically (for example, at an interval of several minutes) acquire time information from upstream wireless devices (such as wireless routers). At a downstream wireless device, an operation (clock updating) is done to correct the device's own time, using the acquired time information. Patent Reference 1 noted below discloses an example of a conventional wireless field device performing the above-described power-saving operation.